Re: BUG #17271: Updating enum columns type fails when constraints exist

"David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
> On Thu, Nov 4, 2021 at 6:54 AM PG Bug reporting form <noreply(at)postgresql(dot)org>
> wrote:
>> ERROR: operator does not exist: direction_state = direction_state_old

> This does not surprise me - why do you believe it is a bug? What should
> happen?

Yeah. Concretely, what you have in the constraint (after the RENAME
TYPE) is

# \d example
...
Check constraints:
"ck_example_direction" CHECK (direction = 'DOWN'::direction_state_old AND below OR NOT below)

Simply updating "direction" to some other type doesn't provide any
guidance about changing the type of the constant, so re-parsing the
constraint fails. It's hard to see what we could do here that
wouldn't involve a lot of guesswork and chances of getting things
wrong.

I concur with David's advice that ALTER TYPE ADD VALUE would
be a better answer, at least for the specific use-case you're
showing here.

regards, tom lane